Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/25—Arrangements specific to fibre transmission
  • H04B10/2581—Multimode transmission
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0278—WDM optical network architectures
  • H04J14/0282—WDM tree architectures
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
  • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
  • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
  • G02B6/2937—In line lens-filtering-lens devices, i.e. elements arranged along a line and mountable in a cylindrical package for compactness, e.g. 3- port device with GRIN lenses sandwiching a single filter operating at normal incidence in a tubular package
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/27—Arrangements for networking
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0227—Operation, administration, maintenance or provisioning [OAMP] of WDM networks, e.g. media access, routing or wavelength allocation
  • H04J14/0228—Wavelength allocation for communications one-to-all, e.g. broadcasting wavelengths
  • H04J14/023—Wavelength allocation for communications one-to-all, e.g. broadcasting wavelengths in WDM passive optical networks [WDM-PON]
  • H04J14/0232—Wavelength allocation for communications one-to-all, e.g. broadcasting wavelengths in WDM passive optical networks [WDM-PON] for downstream transmission
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0227—Operation, administration, maintenance or provisioning [OAMP] of WDM networks, e.g. media access, routing or wavelength allocation
  • H04J14/0241—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths
  • H04J14/0242—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON
  • H04J14/0245—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON for downstream transmission, e.g. optical line terminal [OLT] to ONU
  • H04J14/0246—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON for downstream transmission, e.g. optical line terminal [OLT] to ONU using one wavelength per ONU
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0227—Operation, administration, maintenance or provisioning [OAMP] of WDM networks, e.g. media access, routing or wavelength allocation
  • H04J14/0241—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths
  • H04J14/0242—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON
  • H04J14/0245—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON for downstream transmission, e.g. optical line terminal [OLT] to ONU
  • H04J14/0247—Sharing one wavelength for at least a group of ONUs
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0227—Operation, administration, maintenance or provisioning [OAMP] of WDM networks, e.g. media access, routing or wavelength allocation
  • H04J14/0241—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths
  • H04J14/0242—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON
  • H04J14/0249—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON for upstream transmission, e.g. ONU-to-OLT or ONU-to-ONU
  • H04J14/025—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON for upstream transmission, e.g. ONU-to-OLT or ONU-to-ONU using one wavelength per ONU, e.g. for transmissions from-ONU-to-OLT or from-ONU-to-ONU
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0227—Operation, administration, maintenance or provisioning [OAMP] of WDM networks, e.g. media access, routing or wavelength allocation
  • H04J14/0241—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths
  • H04J14/0242—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON
  • H04J14/0249—Wavelength allocation for communications one-to-one, e.g. unicasting wavelengths in WDM-PON for upstream transmission, e.g. ONU-to-OLT or ONU-to-ONU
  • H04J14/0252—Sharing one wavelength for at least a group of ONUs, e.g. for transmissions from-ONU-to-OLT or from-ONU-to-ONU
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0227—Operation, administration, maintenance or provisioning [OAMP] of WDM networks, e.g. media access, routing or wavelength allocation

Definitions

• the present invention relates to optical network technology and more particularly to a Passive Optical Network (PON) technology which can be used in building Fiber- To-The-Home (FTTH).
• PON Passive Optical Network
• FTTH Fiber-To-The-Home
• IP-TV internet protocol television
• PON Passive Optical Network
• EON Ethernet PON
• GPI gigabit-capable PON
• WDM Wavelength-Division Multiplexing
• 1OG EPON/GPONs capable of supporting gigabit or more data per channel (per home) have been developed as a next- generation FTTH technology.
• TDMA-PON TDMA-PON technology
• a central office or a telephone office
• OLT Optical Line Termination
• ONU Optical Network Unit
• the OLT and the ONU are connected to each other by an optical fiber through an optical splitter, so that uplink and downlink optical signals can be transmitted and received therebetween.
• the ONU is also called an Optical Network Terminal (ONT).
• an uplink wavelength is standardized at 1310nm
• a downlink wavelength is standardized at 1490nm.
• an extra wavelength is used for a video overlay so as to support an existing satellite broadcast service, and a cable television (CATV) service, etc.
• the downlink wavelength for the video overlay has not been laid out yet in the standards document, but uses 1550nm as a provisionally acceptable wavelength.
• standardization of the 1OG EPON has been in progress by IEEE 802.3av
• standardization of 1OG GPON and WDM-PON has been undergoing examination and discussion at the full service access network (FSAN) forum.
• FSAN full service access network
• Fig. 1 shows the foregoing general Time-Division Multiplexing (TDM) Passive Optical Network (PON).
• TDM Time-Division Multiplexing
• PON Passive Optical Network
• an analog data wavelength (1550nm) ⁇ 3 for the video overlay and a digital data wavelength (1490nm) ⁇ 2 generated as a downlink transmission signal in each of the OLTs 30a, 30b are split as many as n optical intensities in the optical splitter l la, 1 Ib via the optical fibers 20a, 20b and then sent to the ONU 10a, 10b.
• the split number n of the optical splitter is '16', '32', or '64' with regard to one OLT.
• a general transmission distance between the OLT and the ONU is about 10km to 20km.
• the optical fiber is installed in the form of a fiber bundle 20 from the central office to a subscriber area, and the optical splitter l la, 1 Ib is provided as a remote node for the optical split corresponding to a plurality of subscribers in an area near the subscribers.
• the present invention is conceived to solve the above problems of the conventional techniques, and an aspect of the present invention to provide a multiple passive optical network that is capable of offering a low speed/low data capacity service and a high speed/high data capacity service.
• Another aspect of the present invention is to provide a Wavelength Division Multiplexing (WDM) coupler that has an improved structure necessary for the multiple passive optical network.
• WDM Wavelength Division Multiplexing
• a second passive optical network providing a high speed/high capacity service is applied to a first passive optical network providing a low speed/low data capacity service, so that not only can a subscriber select their desired network service but also network resources are shared to prevent a resource waste.
• the present invention provides an economical and efficient fiber- to-the-home (FTTH) infrastructure.
• a wavelength to be assigned is easily split and combined in different passive optical networks, so that the different passive optical networks can be applied to each other.
• Fig. 1 shows a general Time-Division Multiplexing (TDM) Passive Optical Network
• FIG. 2 shows a multiple passive optical network according to an exemplary embodiment of the present invention
• FIG. 3 shows the multiple passive optical network according to another exemplary embodiment of the present invention.
• FIG. 4 shows wavelength assignment for the multiple passive optical network according to an exemplary embodiment of the present invention
• FIG. 5 shows an example of a wavelength splitter/combiner
• Fig. 6 shows a wavelength splitter/combiner according to an exemplary embodiment of the present invention
• Fig. 7 shows the wavelength splitter/combiner according to another exemplary embodiment of the present invention.
• Fig. 8 shows the wavelength splitter/combiner according to a third exemplary embodiment of the present invention. Best Mode for Carrying Out the Invention
• PON Network
• first PON and a second PON which provides a service different in speed and capacity from the first PON while partially sharing network resources with the first PON.
• the first PON may include at least one first optical line termination (OLT), a plurality of first optical network units (ONUs) corresponding to the first OLT, and a first remote node which separates a downlink transmission wavelength from the first OLT to the plurality of first ONUs and sends uplink transmission wavelengths from the plurality of first ONUs to the first OLT; and the second PON includes at least one second OLT, a plurality of second ONUs corresponding to the second OLT, and a second remote node which separates a downlink transmission wavelength from the second OLT to the plurality of second ONUs and sends uplink transmission wavelengths from the plurality of second ONUs to the first OLT, wherein an optical fiber is shared in at least one section between a side including the first OLT and the plurality of first ONUs corresponding to the first OLT and a side including the second OLT and the plurality of second ONUs corresponding to the second OLT to transmit and receive the uplink and downlink transmission wavelengths.
• OLT
• the second PON may include: a first splitter/combiner which splits or combines the uplink and downlink transmission wavelengths to be transmitted and received in the first OLT and the uplink and downlink transmission wavelengths to be transmitted and received in the second OLT through the shared optical fiber, and a second splitter/ combiner which splits or combiners the uplink and downlink transmission wavelengths to be transmitted and received in the first ONUs and the uplink and downlink transmission wavelengths to be transmitted and received in the second ONUs through the shared optical fiber.
• a wavelength splitter/ combiner includes: a dual filter including a first thin film filter and a second thin film filter, which are placed symmetrically to each other, to transmit or reflect wavelengths according to the wavelengths; at least one first input/output port placed at a side of the first thin film filter; and at least one second input/output port placed at a side of the second thin film filter, wherein the dual filter includes a wavelength-division multiplexing (WDM) coupler that transmits and reflects uplink and downlink transmission wavelengths assigned to a first passive optical network (PON) and uplink and downlink transmission wavelengths assigned to a second PON different from the first PON, which are incident to the input/output ports, such that the wavelength are output through the corresponding input/output ports to split or combined.
• WDM wavelength-division multiplexing
• Fig. 2 shows a multiple Passive Optical Network (PON) according to an exemplary embodiment of the present invention.
• the multiple passive optical network includes first and second passive optical networks which support services with different speeds and capacities.
• the first PON is a network capable of supporting a low-speed and low-capacity service, and an Ethernet Passive Optical Network (EPON) or a Gigabit-capable Passive Optical Network (GPON) may be employed as the first PON.
• the second PON is a next-generation network capable of supporting a high-speed and high-capacity service, and 1OG EPON, 1OG GPM, or Wavelength-Division Multiplexing (WDM) PON may be used as the second PON.
• WDM Wavelength-Division Multiplexing
• the first PON is illustrated as an E/GPON
• the second PON is illustrated as an N-PON (next-generation PON).
• the N-PON may include a 1OG EPON, 1OG GPON or WDM-PON.
• a central office (CO) 100 includes an Optical Line Termination (OLT) 100a, 110b for the first PON, and an optical line termination (OLT) 120a, 120b for the second PON.
• a subscriber group 200, 300 includes an Optical Network Unit (ONU) 210a, 310a for the first PON, and an optical network unit (ONU) 210b, 310b for the second PON.
• the OLT 110a corresponds to the ONUs 210a, and transmits and receives a wavelength ⁇ l for an uplink transmission signal, a wavelength ⁇ 2 for a downlink transmission signal, and a wavelength ⁇ 3 for a video overlay with respect to the ONUs 210a.
• the OLT 120a transmits and receives a wavelength ⁇ 4 for an uplink transmission signal and a wavelength ⁇ 5 for a downlink transmission signal with respect to the ONUs 210b.
• the OLT 110b corresponds to the ONUs 310a, and transmits and receives the wavelength ⁇ l for the uplink transmission signal, the wavelength ⁇ 2 for the downlink transmission signal, and the wavelength ⁇ 3 for the video overlay with respect to the ONUs 310a.
• the OLT 120b transmits and receives the wavelength ⁇ 4 for the uplink transmission signal and the wavelength ⁇ 5 for the downlink transmission signal with respect to the ONUs 310b.
• the wavelength ⁇ 3 for the video overlay is used in the first PON, but is not limited thereto.
• network resources are partially shared between the first PON and the second PON.
• an optical fiber used as a transmission medium is at least partially shared between the first PON and the second PON.
• a multiple passive optical network system for sharing at least a part of the optical fiber will be described in detail.
• the central office 100 includes a first splitter/combiner 130a, 130b.
• the first splitter/ combiner 130a splits or combines the wavelengths ⁇ l, ⁇ 2 and ⁇ 3 assigned to the first PON and the wavelengths ⁇ 4 and ⁇ 5 assigned to the second PON.
• the first splitter/combiner 130a combines the downlink transmission wavelength ⁇ 2 and the video overlay wavelength ⁇ 3, which are received from the OLT 110a, with the downlink transmission wavelength ⁇ 5, which is received from the OLT 120a, and then sends the combined wavelengths to a first subscriber group 200 through an optical fiber 410 of a fiber bundle 400 in an optical distribution network (ODN).
• ODN optical distribution network
• the first splitter/combiner 130a splits the uplink transmission wavelengths ⁇ l and ⁇ 4 received from the first subscriber 200 through the optical fiber 410, and then sends the wavelength ⁇ l and the wavelength ⁇ 4 to the OLT 110a and the OLT 120a, re- spectively.
• the first splitter/combiner 130b performs the same function as the first splitter/combiner 130a at this location.
• the first subscriber group 200 includes a second splitter/combiner 240.
• the second splitter/combiner 240 splits or combines the wavelengths ⁇ l, ⁇ 2 and ⁇ 3 assigned to the first PON and the wavelengths ⁇ 4 and ⁇ 5 assigned to the second PON.
• the second splitter/combiner 240 combines the uplink transmission wavelength ⁇ l received from a first remote node 220 and the uplink transmission wavelength ⁇ 4 received from a second remote node 230, and sends the combined wavelengths to the central office 100 through the optical fiber 410.
• the second splitter/combiner 240 splits the wavelengths ⁇ 2, ⁇ 3 and ⁇ 5 received through the optical fiber 410, thereby sending the wavelengths ⁇ 2 and ⁇ 3 to the first remote mode 220 and the wavelength ⁇ 5 to the second remote node 230.
• a WDM coupler may be used as the second splitter/combiner 240.
• the first remote node 220 branches the downlink transmission wavelength ⁇ 2 and the video overlay wavelength ⁇ 3, which are split by the second splitter/combiner 240, into a plurality of ONUs 210a, and sends the uplink transmission wavelength ⁇ l from the plurality of ONUs 210a to the second splitter/combiner 240. Further, the second remote node 230 branches the downlink transmission wavelength ⁇ 5, which is split by the second splitter/combiner 240, to a plurality of ONUs 210b, and sends the uplink transmission wavelength ⁇ 4 from the plurality of ONUs 210b to the second splitter/ combiner 240.
• the second PON is a 1OG TDMA-PON (1OG EPON or 1OG GPON)
• a power splitter may be employed as the second remote node 230.
• the second PON is the WDM-PON
• an Arrayed Waveguide Grating (AWG) may be used as the second remote node 230.
• the second subscriber group 300 is different from the first subscriber group 200.
• the ONUs 310a and the ONUs 310b of the second subscriber group 300 operate corresponding to the OLT HOb and the OLT 120b of the central office 100, respectively.
• the first splitter/ combiner 130b of the central office has the same function as the above-described first splitter/combiner 130a, and thus repetitive descriptions thereof will be omitted.
• the second subscriber group 300 includes a remote node 320 and a second splitter/ combiner 330.
• the remote node 320 branches off the downlink transmission wavelengths ⁇ 2 and ⁇ 5 and the video overlay wavelength ⁇ 3, which are received from the central office 100 through the optical fiber 420, and sends the uplink transmission wavelengths ⁇ l and ⁇ 4 from the ONUs 310a and the ONUs 310b at branched locations to the central office 100 through the optical fiber 420.
• the power splitter be used as the remote node 320.
• the second splitter/combiner 330 is provided at each location branched from the power splitter 320, and splits or combines the wavelengths ⁇ l, ⁇ 2 and ⁇ 3 assigned to the first PON and the wavelengths ⁇ 4 and ⁇ 5 assigned to the second PON.
• the second splitter/combiner 330 splits the branched downlink transmission wavelengths ⁇ 2, ⁇ 3 and ⁇ 5, and sends the wavelengths ⁇ 2 and ⁇ 3 to the ONU 310a and the wavelength ⁇ 5 to the ONU 310b.
• the second splitter/combiner 330 combines the uplink transmission wavelength ⁇ l transmitted from the ONU 310a and the uplink transmission wavelength ⁇ 4 transmitted from the ONU 310b, and sends the combined wavelengths to the power splitter 320.
• the first subscriber group 200 employs the WDM coupler 240 provided in front of the first remote node 220 for the first PON so that the wavelengths ⁇ 4 and ⁇ 5 for the second PON are split from the wavelengths ⁇ l, ⁇ 2 and ⁇ 3 for the first PON; and the second subscriber group 300 uses the WDM couplers 330 each located on branches toward the subscribers from the remote node 320 for the first PON so that the wavelengths ⁇ l, ⁇ 2 and ⁇ 3 for the first PON and the wavelengths ⁇ 4 and ⁇ 5 for the second PON are split from each other.
• the first subscriber group 200 is useful if the number of branches from the power splitter 220 is relatively large or if a subscriber group for the second PON is locally separated from a subscriber group for the first PON.
• the second subscriber group 300 is useful to minimize installation of the optical fiber for the second PON from the remote node 320 to the subscriber.
• FIG. 3 shows a multiple passive optical network according to another exemplary embodiment of the present invention.
• Fig. 3 additionally includes a branching unit 140 in the central office 100.
• the branching unit 140 is the power splitter.
• the branching unit 140 branches the downlink transmission wavelength ⁇ 5 to the first splitter/combiner 130a, 130b, and also sends the uplink transmission wavelength ⁇ 4 branched in the first splitter/combiner 130a, 130b to the OLT 120. This is necessary since the OLT 120 of the second PON has a larger subscriber capacity as a next-generation PON. In other words, the number of branches from the branching unit 140 is determined according to performance of the second PON.
• a video overlay function is typically provided along with the EPON/GPON, but may be provided along with the next-generation PON (N-PON), if necessary.
• the WDM couplers 130a, 130b, 240, 330 for combination and division of a wavelength band are necessary for the multiple PON, and they can be manufactured to have an insertion loss within about 0.7dB.
• Fig. 4 shows wavelength assignment for the multiple passive optical network according to an exemplary embodiment of the present invention.
• Fig. 4 is a graph explaining transmission properties of a WDM filter and a wavelength band assignment method for the N-PON adapted to the foregoing multiple PON.
• ⁇ l and ⁇ 2 indicate a standard wavelength band for bidirectional communication defined in the EPON/GPON, in which ⁇ l and ⁇ 2 have center wavelengths of 1310nm and 1490nm, respectively.
• ⁇ 3 for the video overlay is 1550nm, but this has not been stated yet in the EPON/GPON standards document.
• the wavelength for the next-generation (N-PON) has not been determined yet, but has recently undergone discussion in Full Service Access Network (FSAN) and Institute of Electrical and Electronics Engineers (IEEE) while considering several possibilities.
• FSAN Full Service Access Network
• IEEE Institute of Electrical and Electronics Engineers
• ⁇ 4 indicates a certain center wavelength for uplink or downlink transmission selected in the range of 1510nm to 1540nm
• ⁇ 5 indicates a certain center wavelength for uplink or downlink transmission selected in the range of wavelengths longer than ⁇ 3.
• ⁇ 4 is a center wavelength for the uplink transmission
• ⁇ 5 is a center wavelength for the downlink transmission, thereby matching with the foregoing descriptions.
• solid line 500 indicates transmission depending on wavelength, which shows the transmission properties of a WDM coupler for splitting the wavelengths ⁇ l and ⁇ 2 for the EPON/GPON and the wavelengths ⁇ 3, ⁇ 4 and ⁇ 5 for the video overlay.
• dotted line 600 shows the transmission properties of the WDM coupler for splitting the wavelength ⁇ 3 for the video overlay from the wavelengths ⁇ 3, ⁇ 4 and ⁇ 5 represented by the solid line 500 so as to provide a CATV service based on the video overlay to the subscribers of the EPON/GPON.
• the width of transmission and reflection wavelength edges can be adjusted to have a separation of about IOnm.
• the wavelengths ⁇ l and ⁇ 2 input to an input/output port 610 are split into the wavelength ⁇ l and the wavelength ⁇ 2 through a thin film filter 660 via a dual fiber collimator having a dual fiber ferrule 640 and a green lens 650, in which the wavelength ⁇ l is transmitted and the wavelength ⁇ 2 is reflected.
• the reflective wavelength ⁇ 2 passes through the green lens 650 and the dual fiber ferrule 640 again and is then output through an input/output port 620.
• the transmitted wavelength ⁇ l is combined in a fiber collimator having a green lens 670 and a single fiber ferrule 640, and is then output through an input/output port 630.
• the EPON/GPON may use the WDM coupler having a general structure as shown in Fig. 4 along with an optical thin film filter having proper transmission and reflection properties.
• FIG. 6 shows a WDM coupler according to an exemplary embodiment of the present invention.
• the WDM coupler includes a left dual-fiber collimator that has a dual fiber ferrule 760a, a green lens 760b and a fixing tube 770; a dual thin film filter 750 that has thin film filters 750a, 750b formed at opposite sides of a transparent material and having the same properties; a right dual-fiber collimator that has the same structure as the left one; an optical alignment assembling tube 780; and an outer housing 790.
• a moderate amount of epoxy is applied to the surfaces of the green lenses of the dual fiber collimators, and then the left dual-fiber collimator, the dual thin film filter 750, the assembling tube 780, and the right dual- fiber collimator are assembled in order from bottom to top.
• the assembled elements are cured in the state that they are aligned to achieve optimum transmission and reflection properties between the optical fibers, and then they are encased in the outer housing 790 and cured again.
• the assembling method itself is not directly related to the present invention.
• the WDM coupler shown in Fig. 6 is characteristic in that it first has a left and right symmetrical structure with respect to the input/output port and second has a dual filtering structure with respect to the transmission wavelength.
• input/output properties of the wavelength according to the symmetrical structure will be described below.
• the wavelengths ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 and ⁇ 5 are input to the input/output port 730 in the same direction, the wavelengths ⁇ l and ⁇ 2 transit the dual thin film filter 750 and are output to the input/output port 710, and the wavelengths ⁇ 3, ⁇ 4 and ⁇ 5 are reflected from the thin film filter 750b and output to the input/output port 740. Operation is performed in the same way with regard to the reverse direction.
• the WDM coupler provides the dual filtering structure so that the wavelengths ⁇ l, ⁇ 2, ⁇ 3, ⁇ 4 and ⁇ 5 to be split into respective optical fibers are more reliably isolated.
• the reflected wavelengths ⁇ 3, ⁇ 4 and ⁇ 5 are input to the thin film filter 750a, most optical intensities are primarily reflected and the other weak transmission signals are secondarily reflected from the thin film filter 750b, so that the transmission wavelengths ⁇ l and ⁇ 2 are more reliably isolated from the wavelengths ⁇ 3, ⁇ 4 and ⁇ 5, thereby providing excellent noise filtering characteristics.
• the transmission wavelengths ⁇ l and ⁇ 2 have an additional insertion loss within 0.IdB, but isolation characteristics are improved by 3OdB or more as compared with a single filter.
• the WDM coupler of Fig. 6 greatly improves isolation characteristics as compared with the general WDM coupler if the wavelength ⁇ 3 for the video overlay is included in the N-PON or is not in use.
• FIG. 7 shows a WDM coupler according to another exemplary embodiment of the present invention.
• the WDM couplers shown in Fig. 7 are structured for a case in which the wavelength ⁇ 3 (1550nm) for the video overlay is assigned to the first PON as shown in Fig. 3.
• a first WDM coupler has the same structure as shown in Fig. 6.
• the first WDM coupler includes a first thin film filter 850a which transmits the wavelength group ⁇ l and ⁇ 2 and a second thin film filter 850b which reflects the wavelength group ⁇ 3, ⁇ 4 and ⁇ 5 (see solid line 500 in Fig. 4).
• a second WDM coupler has the same structure as shown in Fig. 6 except that a single fiber collimator is provided at a side for combining only the transmission wavelength ⁇ 3.
• the second WDM coupler includes a third thin film filter 890a and a fourth thin film filter 890b which transmit only the wavelength ⁇ 3 disposed between the wavelengths ⁇ 4 and ⁇ 5 like the dotted line 600 illustrated in Fig. 4.
• the long wavelengths ⁇ 3, ⁇ 4 and ⁇ 5 are input to an input/output port 860 of the second WDM coupler, only the wavelength ⁇ 3 for the video overlay is transmitted through the third and fourth thin film filters 890a and 890b and sent to the first WDM coupler via an input/output port 880, and the wavelengths ⁇ 4 and ⁇ 5 are reflected from the third thin film filter 890a and then output to an input/output port 870.
• the wavelength ⁇ 3 transmitted to the input/output port 840 of the first WDM coupler through the input/output port 880 is reflected from the fourth thin film filter 850b and then output along with the wavelengths ⁇ l and ⁇ 2 for the EPON/GPON to the input/output port 830.
• FIG. 8 shows a WDM coupler according to a third exemplary embodiment of the present invention.
• the WDM couplers shown in Fig. 8 are structured for the case in which the wavelength ⁇ 3 (1550nm) for the video overlay is assigned to the first PON.
• a first thin film filter 940a and a second thin film filter 940b of a first WDM coupler have transmission properties contrary to solid line 500 shown in Fig. 4.
• a third thin film filter 990a and a second thin film filter 990b of a second WDM coupler transmit only the wavelength ⁇ 3 for the video overlay but reflect the other wavelengths ⁇ l, ⁇ 2, ⁇ 4 and ⁇ 5 like dotted line 600 shown in Fig. 4.
• the wavelengths ⁇ 4 and ⁇ 5 are reflected from the fourth thin film filter 990b and output to the input/output port 980.
• the present invention can be effectively applied to an industrial field relating to a multiple passive optical network that is capable of offering a low speed/low data capacity service and a high speed/high data capacity service.

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• 2007
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